Automatic sub-millisecond clock synchronization

ABSTRACT

According to one aspect, embodiments of the invention provide a system for monitoring a plurality of circuit branches coupled to an input line, the system comprising a communication bus, a controller having a primary clock with a first clock value and configured to sample voltage on the input line based on the first clock value, a plurality of sensor circuits, each sensor circuit having a secondary clock with a second clock value and configured to sample current in the at least one of the plurality of circuit branches based on the second clock value, and wherein the controller is further configured to initiate, via the communication bus, synchronization of at least one secondary clock and the primary clock, and to synchronize, via the communication bus, the at least one secondary clock and the primary clock to account for transmission latency in the communication bus.

BACKGROUND OF INVENTION

1. Field of the Invention

At least one example in accordance with the present invention relatesgenerally to systems and methods for monitoring a load center forcurrent, power and energy usage.

2. Discussion of Related Art

A load center or panelboard is a component of an electrical supplysystem which divides an electrical power feed from a power line intodifferent subsidiary circuit branches. Each subsidiary circuit branchmay be connected to a different load. Thus, by dividing the electricalpower feed into subsidiary circuit branches, the load center may allow auser to individually control and monitor the current, power and energyusage of each load.

Current sensors can be used to monitor activity of a load center. Forexample, Current Transformers (CT) are commonly used to monitor currentin a subsidiary or main branch of a load center while maintainingelectrical isolation from the branch. A CT measures current in a branchby producing a reduced current signal, proportionate to the current inthe branch. Based on the generated reduced current signal, the level ofcurrent in the subsidiary branch may be determined The generated signalmay also be further manipulated and measured to assist in efficientenergy management.

SUMMARY OF THE INVENTION

Aspects in accord with the present invention are directed to a systemfor monitoring a plurality of circuit branches coupled to an input line,the system comprising a communication bus, a controller having a primaryclock with a first clock value, the controller configured to be coupledto the communication bus and the input line and further configured tosample voltage on the input line based on the first clock value, aplurality of sensor circuits, each sensor circuit having a secondaryclock with a second clock value and each sensor circuit configured to becoupled to the communication bus and at least one of the plurality ofcircuit branches, wherein each sensor circuit is further configured tosample current in the at least one of the plurality of circuit branchesbased on the second clock value, and wherein the controller is furtherconfigured to initiate, via the communication bus, synchronization of atleast one secondary clock and the primary clock, and to synchronize, viathe communication bus, the at least one secondary clock and the primaryclock to account for transmission latency in the communication bus.

According to one embodiment, the controller is further configured toutilize a multi-drop master-slave communication protocol to communicatewith the plurality of sensor circuits via the communication bus. Inanother embodiment, in initiating synchronization of the at least onesecondary clock and the primary clock, the controller is furtherconfigured to transmit a measurement signal to at least one sensorcircuit having the at least one secondary clock and to start a timerhaving an elapsed time value upon transmitting the measurement signal.In one embodiment, the at least one sensor circuit is further configuredto receive the measurement signal and transmit a response to themeasurement signal to the controller. In another embodiment, the atleast one sensor circuit is further configured to adjust the secondclock value of the secondary clock based on the measurement signal.

According to another embodiment, the controller is further configured toreceive the response to the measurement signal from the at least onesensor circuit, stop the timer in response to receiving the response tothe measurement signal, and calculate at least one Return Trip Time(RTT) based on the elapsed time value of the timer. In anotherembodiment, the controller is further configured to calculate arepresentative RTT based on the at least one RTT. In one embodiment, therepresentative RTT is one of a median RTT, mean RTT, and maximum RTT.

According to one embodiment, the controller is further configured totransmit a first synchronization signal based on the representative RTTto the at least one sensor circuit, and wherein the at least one sensorcircuit is further configured to adjust the second clock value of thesecondary clock based on the first synchronization signal. In oneembodiment, the at least one sensor circuit is further configured toadjust a millisecond counter of the secondary clock based on the firstsynchronization signal. In another embodiment, the controller is furtherconfigured to transmit a second synchronization signal based on therepresentative RTT to the at least one sensor circuit, and wherein theat least one sensor circuit is further configured to adjust the secondclock value based on the second synchronization signal. In oneembodiment, the at least one sensor circuit is further configured toadjust a microsecond counter of the secondary clock based on the secondsynchronization signal.

According to another embodiment, the controller is further configured tosynchronize, via the communication bus, current sampling performed bythe plurality of sensor circuits with the voltage sampling performed bythe controller.

Another aspect in accord with the present invention is directed to amethod for monitoring a plurality of circuit branches coupled to a powerline, the method comprising coupling a controller to the communicationbus and to the power line, the controller having a primary clock with afirst clock value, coupling a sensor circuit to each one of theplurality of circuit branches and to a communication bus, each sensorcircuit having a secondary clock with a second clock value, sampling,with at least one of the sensor circuits, current in at least one of theplurality of circuit branches based on the second clock value, sampling,with the controller, voltage on the power line based on the first clockvalue, and synchronizing, with the controller via the communication bus,the at least one secondary clock and the primary clock to account fortransmission latency in the communication bus.

According to one embodiment, synchronizing the at least one secondaryclock and the primary clock includes calculating at least one RTT fromthe controller to at least one sensor circuit having the at least onesecondary clock, and transmitting at least one synchronization signalfrom the controller to the at least one sensor circuit to adjust thesecond clock value of the secondary clock of the at least one sensorcircuit based on the at least one RTT.

According to another embodiment, calculating at least one RTT includestransmitting a measurement signal from the controller to the at leastone sensor circuit, in response to transmitting the measurement signal,starting a timer of the controller having an elapsed time value,receiving, with the at least one sensor circuit, the measurement signal,transmitting, in response to receiving the measurement signal, aresponse to the measurement signal from the at least one sensor circuitto the controller, receiving, with the controller, the response to themeasurement signal, stopping, in response to receiving the response tothe measurement signal, the timer of the controller, and calculating theat least one RTT based on the elapsed time value of the timer. In oneembodiment, the method further comprises calculating a representativeRTT based on a plurality of RTT calculations, and wherein transmittingat least one synchronization signal from the controller to the at leastone sensor circuit includes transmitting at least one synchronizationsignal from the controller to the at least one sensor circuit to adjustthe second clock value of the at least one sensor circuit based on therepresentative RTT.

According to one embodiment, the method further comprises synchronizing,with the controller via the communication bus, current samplingperformed by the plurality of sensor circuits with the voltage samplingperformed by the controller. In one embodiment, transmitting at leastone synchronization signal from the controller to the at least onesensor circuit to adjust the second clock value of the at least onesensor circuit based on the at least one RTT includes utilizing amulti-drop master-slave communication protocol to transmit the at leastone synchronization signal from the controller to the at least onesensor circuit.

One aspect in accord with the present invention is directed to a systemfor monitoring a plurality of circuit branches coupled to an input line,the system comprising a communication bus, a controller having a primaryclock having a first clock value, the controller configured to becoupled to the communication bus and the input line and furtherconfigured to sample voltage on the input line based on the first clockvalue, a plurality of sensor circuits, each sensor circuit having asecondary clock with a second clock value and each sensor circuitconfigured to be coupled to the communication bus and at least one ofthe plurality of circuit branches, wherein each sensor circuit isfurther configured to sample current in the at least one of theplurality of circuit branches based on the second clock value, and meansfor synchronizing, with the controller via the communication bus, thesecondary clock of at least one of the plurality of sensor circuits andthe primary clock of the controller to within 0.1 millisecond.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various FIGs. is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 is a circuit diagram of a load center in accordance with aspectsof the present invention;

FIG. 2A is a schematic diagram of a smart CT prior to being coupled to acircuit branch in accordance with aspects of the present invention;

FIG. 2B is a schematic diagram of a smart CT after being coupled to acircuit branch in accordance with aspects of the present invention;

FIG. 3A is a schematic diagram of a smart CT prior to being coupled to acommunication bus in accordance with aspects of the present invention;

FIG. 3B is a schematic diagram of a smart CT after being coupled to acommunication bus in accordance with aspects of the present invention;

FIG. 3C is a schematic diagram of a smart CT locked together with acommunication bus in accordance with aspects of the present invention;

FIG. 4 is a circuit diagram of smart CT's coupled to a daisy chain busin accordance with aspects of the present invention;

FIG. 5 is a block diagram of a concentrator in accordance with aspectsof the present invention;

FIG. 6 is a flow chart of a method of operation of a CT concentrator inaccordance with aspects of the present invention; and

FIG. 7 is a flow chart illustrating a method of clock synchronizationbetween a CT concentrator and smart CT's in accordance with aspects ofthe present invention.

DETAILED DESCRIPTION

Examples of the methods and systems discussed herein are not limited inapplication to the details of construction and the arrangement ofcomponents set forth in the following description or illustrated in theaccompanying drawings. The methods and systems are capable ofimplementation in other embodiments and of being practiced or of beingcarried out in various ways. Examples of specific implementations areprovided herein for illustrative purposes only and are not intended tobe limiting. In particular, acts, components, elements and featuresdiscussed in connection with any one or more examples are not intendedto be excluded from a similar role in any other examples.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. Any references toexamples, embodiments, components, elements or acts of the systems andmethods herein referred to in the singular may also embrace embodimentsincluding a plurality, and any references in plural to any embodiment,component, element or act herein may also embrace embodiments includingonly a singularity. References in the singular or plural form are notintended to limit the presently disclosed systems or methods, theircomponents, acts, or elements. The use herein of “including,”“comprising,” “having,” “containing,” “involving,” and variationsthereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.In addition, in the event of inconsistent usages of terms between thisdocument and documents incorporated herein by reference, the term usagein the incorporated references is supplementary to that of thisdocument; for irreconcilable inconsistencies, the term usage in thisdocument controls.

As discussed above, CT's may be utilized with a load center of anelectrical supply system to monitor circuit branches and assist inproviding efficient energy management. For instance, CT's may be coupledto circuit branches inside or outside of a load center. However,multiple challenges with CT's may arise as the electrical supply systemgrows in size and complexity.

Existing methods and systems typically rely on a system of individualCT's, each connected to a main controller and measurement unit in a “huband spoke” topology. In such a system, each CT requires dedicatedcabling connecting it to the main controller and its measurement unit,so that the number of cables or wires increases linearly with the numberof sensors. In addition, some jurisdictions have regulatory requirementson the amount of “gutter space” (i.e., space within the panelboard freeof wiring and other electronic devices) available within a panelboard.Therefore, as the number of CT's increases, the amount of cabling andcircuitry within a panelboard may become difficult to manage and violateregulatory requirements.

In some instances it may even be difficult to physically place all ofthe desired CT's and corresponding circuitry within the load center, anddue to the complexity of such a load center; installation, expansion andmaintenance may also be expensive, difficult and even hazardous.

At least some embodiments described herein overcome these problems andprovide a relatively small, less complex and more manageable method andsystem for utilizing CT's to monitor circuit branches of a load center.

FIG. 1 shows a load center 100 that includes a system for monitoringsubsidiary circuit branches 102 of the load center 100 according to oneembodiment of the current invention. The load center 100 includes ahousing 101. Within the housing 101, the load center 100 includes afirst input power line 104, a second input power line 106, a pluralityof circuit branches 102, a neutral line 108, and a ground connection110. The first and second input power lines 104, 106 are each configuredto be coupled to an external power source (e.g., a utility power system)(not shown). Each one of the plurality of circuit branches 102 isconfigured to be coupled between one of the input power lines 104, 106and an external load 112 (e.g., an appliance, a power outlet, a lightetc.). According to one embodiment, each one of the input power lines104, 106 includes a circuit breaker 113 coupled between the input powerline 104, 106 and circuit branches 102. According to another embodiment,each one of the plurality of circuit branches 102 includes a circuitbreaker 115 coupled between the input power line 104, 106 and anexternal load 112. In one embodiment, the current rating of each of thecircuit breakers 113, 115 may be configured based on the power requiredby the external load 112 to which the circuit breakers 113, 115associated circuit branch 102 is coupled. The neutral line 108 iscoupled to the ground connection 110. According to one embodiment, theneutral line is coupled to the ground connection 110 via a neutral busbar 116. According to another embodiment, the ground connection 110 iscoupled to the neutral line 108 via a ground bus bar 118.

Within the housing 101, the load center 100 also includes a plurality ofCurrent Transformers (CT) 114, a plurality of smart sensor circuits 120,a communication bus 122, and a CT concentrator 124. According to oneembodiment, the communication bus 122 includes a plurality of wires. Forexample, in one embodiment, the communication bus 122 is a ribbon cableincluding 4 wires (a power line, a return line, D+ differential pairline, D− differential pair line); however, in other embodiments, thecommunication bus 122 may include any number and type of wires. Each oneof the plurality of CT's 114 is coupled to at least one of the pluralityof circuit branches 102. According to one embodiment, CT's 114 may alsobe coupled to each input line 104, 106. According to one embodiment,each CT 114 encompasses a corresponding circuit branch 102 or input line104, 106. Each one of the plurality of CT's is also coupled to acorresponding smart sensor circuit 120. Each smart sensor circuit 120 iscoupled to the communication bus 122.

According to one embodiment, each smart sensor circuit 120 is connectedto the communication bus 122 so that each smart sensor circuit 120 is inelectrical communication with the CT concentrator 124. In oneembodiment, each smart sensor circuit 120 is clamped onto thecommunication bus 122. For example, in one embodiment, electricalcontacts (not shown) of a smart sensor circuit 120 are pressed onto thecommunication bus 122 so that the electrical contacts pierce aninsulation layer of the communication bus 122 and become electricallycoupled to appropriate conductors within the communication bus 122. Inother embodiments, the smart sensor circuits 120 may be coupleddifferently to the communication bus 122. For example, according to oneembodiment, the smart sensor circuits 120 may be coupled to thecommunication bus 122 via a bus bar or daisy chained connectors (notshown).

The connection of smart sensor circuits 120 to the communication bus 122is discussed in greater detail below.

According to one embodiment, the CT concentrator 124 includes a digitalinterface 125, at least one analog interface 127, a power module 126 anda Zigbee RF interface 128. The communication bus 122 is coupled to thedigital interface 125. The power module 126 is coupled to at least oneinput power line 104, 106 via at least one branch circuit 102. Accordingto one embodiment (not shown), at least one CT 114 is coupled directlyto at least one analog interface 127.

According to one embodiment, AC power is provided from an externalsource (e.g., a utility power system) to the input lines 104, 106. ACpower from the input lines 104, 106 is provided to each of the externalloads 112, via the circuit branches 102. The circuit breakers 113 areconfigured to automatically open and prevent current in an input line104, 106 if an overload or short circuit is detected in the input line104, 106. The circuit breakers 115 are configured to automatically openand prevent current in a circuit branch 102 if an overload or shortcircuit is detected in the circuit branch 102.

The power module 126 of the CT concentrator 124 receives AC power fromat least one input line 104, 106. Using the AC power, the power module126 powers the CT concentrator 124. In addition, the CT concentrator 124measures the AC voltage, frequency and/or phase of the AC power.According to one embodiment, the CT concentrator 124 is configured tocommunicate the measured AC voltage, frequency and/or phase informationto the smart sensor circuits 120, via the communication bus 122. Forexample, in one embodiment, the CT concentrator 124 transmits phaseinformation of the AC power and/or timing/clock information to the smartsensor circuits 120 so that the CT concentrator 124 may be synchronizedwith the smart sensor circuits 120. The synchronization of the CTconcentrator 124 with the smart sensor circuits 120 will be discussed ingreater detail below. According to one embodiment, the CT concentratoris also capable of being powered by a battery.

AC current passing through a circuit branch 102 or input line 104, 106induces a proportionate AC current in its associated CT 114 whichencompasses the circuit branch 102 or input line 104, 106. According toone embodiment, where a CT 114 may be coupled to multiple circuitbranches 102, an AC current proportionate to the combined current in themultiple circuit branches is induced in the CT 114 which encompasses themultiple circuit branches.

The smart sensor circuit 120 coupled to the CT 114 converts theproportionate AC current from the CT 114 into a digital value and thentransmits the digital value, over the communications bus 122 to the CTconcentrator 124. In addition, the smart sensor circuit 120 may beconfigured to utilize the voltage, frequency and/or phase informationreceived from the CT concentrator 124 over the communications bus 122.For example, in one embodiment, the smart sensor circuit 120 utilizesphase information and/or timing/clock information received from the CTconcentrator 124 to synchronize operation with the CT concentrator 124such that current measurements performed by the smart sensor circuits120 can by synchronized with voltage measurements made by the CTconcentrator 124.

In another example, the smart sensor circuit 120 utilizes the voltage,frequency and/or phase information to calculate power and energyinformation such as RMS current, true and apparent power, and powerfactor of the circuit branch 102 or input line 104, 106. Thisinformation is also converted into digital values and sent to thedigital interface 125 of the CT concentrator 124 over the communicationsbus 122. According to one embodiment, at least one CT 114 may alsoprovide analog signals, proportionate to the AC current passing throughthe circuit branch 102, directly to an analog interface 127 of the CTconcentrator 124.

According to one embodiment, upon receiving the current information fromthe smart sensor circuits 120, the CT concentrator 124 utilizes themeasured voltage, frequency and/or phase information to calculate powerand energy information such as RMS current, true and apparent power, andpower factor of the circuit branch 102 or input line 104, 106.

According to one embodiment, upon receiving the current information andreceiving and/or calculating the power information, the CT concentrator124 transmits the current, power and energy information to an externalclient (e.g., a web server, in-home display, internet gateway etc.) viathe wireless Zigbee RF interface 128 to assist in power management ofthe load center 100 and to assist in power management and control of aresidence or other facility containing the system. The CT concentrator124 may also transmit the current, power and energy information to anexternal client via a wired connection or a different type of wirelessconnection.

By including a single communication bus 122 to which all smart sensorcircuits 120 are coupled, a relatively small, less complex and moremanageable method and system for utilizing a plurality of CT's 114 tomonitor circuit branches 102 of a load center 100 is provided.

FIGS. 2A and 2B illustrate one embodiment of the process of coupling aCT 114 to a circuit branch 102. According to one embodiment, a housing205 includes a CT 114 and a smart sensor circuit 120 enclosed therein.In one embodiment, a first portion 214 of the housing 205 includes a CT114 and a second portion 216 includes a smart sensor circuit 120. FIG.2A illustrates the first portion 214 prior to being coupled to a circuitbranch 102 and FIG. 2B illustrates the first portion 214 after beingcoupled to a circuit branch 102.

The first portion 214 is coupled to the second portion 216 via a hinge206. The second portion 216 includes a button 202 coupled to a lever204. Prior to the first portion 214 being coupled to the circuit branch102, the lever 114 is in an upward position, allowing the first portion214 to swing away from the second portion 216 and create an opening 208by which a circuit branch 102 may be inserted. When connection to acircuit branch 102 is desired, a user may configure the first portion214 so that the circuit branch 102 is inserted through the opening 208into an interior chamber 209. The user may then press down on the button202, causing the lever 204 to move in a downwards direction. The lever204 presses against an outside portion 210 of the first portion 214,causing the first portion 214 to swing towards the second portion 216and capture the circuit branch 102 within the interior chamber 209 ofthe first portion 214. According to other embodiments, the first portion214 may be connected to the circuit branch 102 differently. For example,the first portion 214 may be manually placed around the circuit branch102. As discussed above, after the circuit branch 102 is encompassed bythe first portion 214 (and hence also the CT 114), an AC current in thecircuit branch 102 will produce a proportionate AC current within the CT114.

FIGS. 3A, 3B and 3C illustrate the process of coupling the secondportion 216 to a communications bus 122. FIG. 3A illustrates the secondportion 216 prior to being connected to a communications bus 122. FIG.3B illustrates the second portion 216 after being connected to acommunication bus 122. FIG. 3C illustrates the second portion 216 lockedtogether with a communications bus 122. According to one embodiment, thesecond portion 216 includes an Insulation Displacement Connector (IDC)302 (e.g., an AVX series 9176 IDC). According to one embodiment, the IDC302 may include a plurality of blades 304. For example, if, as discussedabove, the second portion 216 (and hence the smart sensor circuit 120)is configured to be coupled to a four-wire ribbon cable, the IDC 302will include four blades, each blade configured to be coupled to acorresponding conductor within the cable. However, according to otherembodiments, the IDC 302 may include any number of blades to adequatelyconnect the smart sensor circuit 120 to the communications bus 122.

The second portion 216 may also include a locking lid 306 coupled to thesecond portion 216 via a hinge 308. Prior to being coupled to thecommunications bus 122, the locking lid 306 of the second portion 216 isswung away from the IDC 302, allowing a user to place the communicationbus 122 adjacent to the IDC 302. The user presses down on thecommunication bus 122, causing the communication bus 122 to pressagainst the IDC 302. The plurality of blades 304 of the IDS 302 piercethe outer insulation layer 310 of the communication bus 122, each one ofthe plurality of blades 304 connecting with a corresponding conductorwithin the communication bus 122. The user may then swing the lockinglid towards the IDC 302 and press down on the locking lid to lock thecommunication bus 122 into place. According to other embodiments, thesecond portion 216 (and hence the smart sensor circuits 120) may becoupled to the communication bus 122 in a different manner. For example,smart sensor circuits may also be coupled to the communication bus 122via a bus bar. Upon being coupled to the communication bus 122, thesmart sensor circuit 120 is in electrical communication with the CTconcentrator 124.

FIG. 4 is a circuit diagram of a plurality of CT's 114 and smart sensorcircuits 120 coupled to a communication bus 122. Each CT 114 is coupledto a circuit branch 102, or input line 104, 106, as discussed above. Forexample, in one embodiment each CT 114 is configured to encompass acircuit branch 102, or input line 104, 106, as discussed in relation toFIGS. 2A and 2B. Each smart sensor circuit 120 is coupled to acommunication bus 122 as discussed above. According to one embodiment,the communication bus 122 may be a 4-wire ribbon cable including a powerline 122 d, a D− differential pair line 122 c, a D+ differential pairline 122 b, and a return (ground) line 122 a. In one embodiment, thecommunication bus 122 is a RS-485 bus; however, according to otherembodiments, a different type of bus may be used.

Each smart sensor circuit 120 includes a microcontroller 402. In oneembodiment, the microcontroller 402 is a low power microcontroller(e.g., an STM8 low power microcontroller). According to one embodiment,the microcontroller 402 includes an analog interface 404, a referenceinterface 406, a power interface 408, a return interface 410, atransmission interface 412 and a reception interface 414. According toone embodiment, the power interface 408 is coupled to the power line 122d and the return interface 410 is coupled to the return line 122 a. Inthis way, each smart sensor circuit 120 is powered by the communicationbus 122. According to another embodiment, each CT 114 is coupled inparallel between the analog interface 404 and the reference interface406. In one embodiment, each smart sensor circuit 120 also includes aburden resistor 415 coupled in parallel between the analog interface 404and the reference interface 406.

Each smart sensor circuit 120 also includes a transceiver 403 (e.g., anRS-485 Transceiver). According to one embodiment, the transceiver 403includes a first diode 416 coupled between the transmission interface412 and the communication bus 122, and a second diode 418 coupledbetween the reception interface 414 and the communication bus 122. Also,in one embodiment, the transceiver 403 is coupled in parallel betweenthe power 122 d and return 122 a lines.

As discussed previously, AC current 416 in the circuit branch 102 orinput line 104, 106 to which a CT 114 is coupled, will produce aproportionate AC current 418 in the CT 114. The burden resistor 415converts the proportionate AC current 418 into a proportionate ACvoltage. Via the analog interface 404, the microcontroller 402 receivesthe proportionate AC voltage and converts the proportionate AC voltageinto a digital value. The microcontroller 402 then provides the digitalvalue to the transmission line 122 b via the transmission interface 412and transceiver 403, and transmits the digital value over thecommunication bus 122 to the CT concentrator 124. In addition, themicrocontroller 402 is configured to receive voltage, frequency and/orphase information from the CT concentrator 124, via the reception line122 c, the transceiver 403 and the reception interface 414. As discussedabove, the microcontroller 402 may use the additional voltage, frequencyand/or phase information received from the CT concentrator 124 alongwith the received proportionate AC current 418 to calculate power andenergy information of the circuit branch 102 or input line 104, 106 suchas RMS current, true and apparent power, and power factor. Thisinformation may also be converted into digital values and transmitted tothe CT concentrator 124 via the transmission interface 412, thetransceiver 403 and the transmission line 122 b. In one embodiment, themicrocontroller 402 may also use phase information and/or timing/clockinformation received from the CT concentrator 124 to synchronize currentmeasurements in the smart sensor circuits 120 with voltage measurementsin the CT concentrator 124

FIG. 5 is a block diagram of a CT concentrator 124. As discussed above,the CT concentrator 124 has a digital interface 125 coupled to thecommunication bus 122. The communications bus is coupled to a pluralityof smart sensor circuits 120 and a plurality of CT's 114.

According to one embodiment, the CT concentrator 124 includes a powermodule 126. In one embodiment, the power module 126 includes asingle-phase power interface 502 configured to be coupled to asingle-phase power supply. In another embodiment the power module 126includes a three-phase power interface 504 configured to be coupled to athree-phase power supply. For example, the three-phase power interface504 may be configured to receive power from a 3-phase delta or wye powerconnection. It is to be appreciated that the power supply coupled to thesingle-phase 502 or three-phase 504 interface is the same power supplycoupled to the input lines 104, 106 and as described in relation toFIG. 1. Accordingly, power received by the power module 126 issubstantially the same as power being provided to the circuit branches102.

According to one embodiment, the power module 126 also includes a DCinterface 506, a sensor interface 508 and an extra pin interface 510.According to one embodiment, the extra pin interface 510 includes fouradditional pins (e.g., a transmission pin, a reception pin, a powermodule type pin and an auxiliary power pin). However, in otherembodiments, the extra pin interface 510 may include any number and typeof pins. According to another embodiment, the CT concentrator 124 mayalso include a battery pack 512 having a DC interface 514. In oneembodiment, the power module 126 and/or battery pack 512 is modular andmay be removed from the CT concentrator 124.

According to one embodiment, the CT concentrator 124 includes a first DCinterface 516 configured to be coupled to the DC interface 514 of thebattery pack 512, a second DC interface 518 configured to coupled to theDC interface 506 of the power module 126, a sensor interface 520configured to be coupled to the sensor interface 508 of the power module126, and an extra pin interface 522 configured to be coupled to theextra pin interface 510 of the power module 126. The extra pin interface522 includes four additional pins (e.g., a transmission pin, a receptionpin, a power module type pin and an auxiliary power pin). However, inother embodiments, the extra pin interface 522 may include any numberand type of pins.

The first 516 and second 518 DC interfaces are coupled to a powermanagement module 524. The power management module 524 is coupled to amicrocontroller 528. The sensor interface 520 and the extra pininterface 522 are coupled to the microcontroller 528. The CTconcentrator 124 also includes a transceiver 530 coupled between thedigital interface 125 and the microcontroller 528 and a non-volatilememory module 532 coupled to the microcontroller 528. In one embodiment,the non-volatile memory module 532 includes Electrically ErasableProgrammable Read-Only Memory (EEPROM); however, in other embodiments,the non-volatile memory module 532 may include any type of non-volatilememory (e.g., such as serial Flash memory).

The CT concentrator 124 also includes a user interface 534 coupled tothe microcontroller. In some embodiments, the user interface may includeany type of controls which allows a user to interface with the CTconcentrator 124. (e.g., such controls include switches, buttons, LED'setc.). According to one embodiment, the CT concentrator 124 alsoincludes a USB port 536 and a serial port 538.

The CT concentrator 124 also includes a wireless radio module andantenna 540. In one embodiment, the wireless radio module is a ZigBeeradio; however, in other embodiments, the wireless radio module 540 maybe configured using a different wireless standard. According to oneembodiment, the wireless radio and antenna 540 is coupled to themicrocontroller 528, an On/Off switch 542, and a serial memory module544.

The power module 126 receives AC power from a power source (e.g., asingle-phase or three phase power source) (not shown), modulates andconverts the received AC power to DC power, and provides DC power to theCT concentrator 124 via the DC interface 506 and the second DC interface518. The power management module 524 receives the DC power from thesecond DC interface 518 and provides appropriate DC power to componentsof the CT concentrator 124 (e.g., the microcontroller 528). According toanother embodiment, the battery pack 512 may provide DC power to the CTconcentrator 124 via the DC interface 514 and the first DC interface516. The power management module 524 receives the DC power from thefirst DC interface 516 and provides appropriate DC power to componentsof the CT concentrator 124 (e.g., the microcontroller 528).

The power module 126 provides power signals received from the powersource (e.g., single-phase or three-phase source) to the microcontroller528 via the sensor interfaces 508, 520. In one embodiment, the powersignals include a voltage sense signal and a phase synchronizationsignal. According to another embodiment, the power module 126 alsoprovides additional information to the microcontroller via the extra pininterfaces 510, 522. For example, additional information may be providedto the microcontroller via a transmission pin, a reception pin, a powermodule type pin and an auxiliary power pin.

The microcontroller 528 receives the power signal information from thepower module 126, via the sensor interface 520. The microcontroller 528measures the voltage, frequency and phase of the power being provided tothe power module 126. It is to be appreciated that as the power providedto the power module 126 is substantially the same as power provided tothe circuit branches 102 (as discussed above), the voltage, frequencyand phase measured by the microcontroller 528 in relation to the powermodule 126 is the same as the voltage, frequency and phase of the powerbeing provided to the circuit branches 102.

Upon being powered, the microcontroller 528 begins to communicate withthe smart sensor circuits 120 via the transceiver 530, the digitalinterface 125 and the communication bus 122. According to oneembodiment, the microcontroller 528 may utilize the RS-485 physicalcommunication protocol to communicate over the communication bus 122.However, other physical communication protocols may be used. Themicrocontroller 528, which acts as the primary controller, identifieswhich smart sensor circuits 120 are coupled to the communication bus122. The primary microcontroller 528 treats the microcontrollers 402 assecondary controllers and assigns each secondary microcontroller 402(and hence smart sensor circuit 120) a unique address. According to oneembodiment, each time a new smart sensor circuit 120 is coupled to thecommunication bus 122, it is assigned a new address by the primarymicrocontroller 528.

According to one embodiment, the primary microcontroller 528 utilizesthe Modbus serial communication protocol to define the communication andaddressing on the communication bus 122. The primary microcontroller528, using the Modbus protocol, assigns unique addresses to the smartsensor circuits 120 and sets the structure and format of the data thatis transmitted over the communication bus 122. For example, according toone embodiment, communication over the communication bus 122 using theModbus protocol may be performed as described in U.S. patent applicationSer. No. 13/089,686 entitled “SYSTEM AND METHOD FOR TRANSFERRING DATA INA MULTI-DROP NETWORK”, filed on Apr. 19, 2011, which is hereinincorporated by reference in its entirety. In one embodiment, theprimary microcontroller 528 utilizes an auto addressing scheme. Forexample, the primary microcontroller 528 utilizes an auto addressingscheme as described in U.S. patent application Ser. No. 13/089,678entitled “SYSTEM AND METHOD FOR AUTOMATICALLY ADDRESSING DEVICES IN AMULTI-DROP NETWORK”, filed on Apr. 19, 2011, which is hereinincorporated by reference in its entirety.

According to one embodiment, the Modbus protocol allows for up to 255smart sensor circuits 120 to be simultaneously attached to thecommunication bus 122. It also is to be appreciated that the number ofsmart sensor circuits 120 may be limited by the load center 100 itself.For example, in common residential load centers, the maximum number ofbranch circuits (and hence smart sensor circuits) is seventy-two.However, according to at least one embodiment, different communicationprotocols may be used by the primary 528 and secondary 402microcontrollers to allow any number of smart sensor circuits 120 to becoupled to the communication bus 122 (e.g., for use in large, commercialload centers).

According to one embodiment, once all of the smart sensor circuits 120have been identified and assigned addresses by the primarymicrocontroller 528, a user, via the user interface 534, may associateeach smart sensor circuit 120 with a specific load (e.g., sensor #12 isassigned to an air conditioner; sensor #13 is assigned to aRefrigerator, etc.).

Once the identification and addressing of the smart sensor circuits 120is complete, the primary microcontroller 528 controls the smart sensorcircuits 120. The primary microcontroller 528 controls communication onthe bus 122 to eliminate conflicts or data collision. In addition,according to one embodiment, the primary microcontroller 528 providespower related information or data to the smart sensor circuits 120. Forexample, as discussed above, the primary microcontroller 528 measuresthe voltage, frequency and phase of the power being provided to thepower module 126 (and hence the circuit branches 102). When needed by asmart sensor circuit 120, the primary microcontroller 528 transmits thepower related information (and/or other appropriate information such astiming/clock information) to the smart sensor circuit 120, via thetransceiver 530 and communication bus 122.

As discussed above, each smart sensor circuit 120 measures the currentthrough an associated circuit branch 102 or input line 104, 106.According to one embodiment, using the measured current and the receivedadditional power related information (e.g., voltage, frequency andphase) from the primary microcontroller 528, a smart sensor circuit 120calculates power information such as RMS current, true and apparentpower, and power factor of the associated circuit branch 102 or inputline 104, 106. The calculated current and/or power information istransmitted to the primary microcontroller 528, via the communicationbus 122, digital interface 125, and transceiver 530. In one embodiment,the power information is transmitted to the primary microcontroller 528at a time and rate determined by the microcontroller 528.

According to one embodiment, upon receiving the calculated current fromthe smart sensor circuits 120, the primary microcontroller 528 utilizesthe measured voltage, frequency and/or phase information to calculatepower and energy information such as RMS current, true and apparentpower, and power factor of the circuit branch 102 or input line 104,106.

The current, power and energy information is provided to the wirelessradio module and by the primary microcontroller 528. The wireless radiomodule wirelessly transmits (via the antenna 540) the current, power andenergy information to an external client (e.g., a web server, in-homedisplay, or internet gateway) to provide electric power and energyconsumption data to end users or other interested parties. According toone embodiment, the current, power and energy information may also beprovided to an external client through a wired connection (e.g., via theUSB port 536 or serial port 538). According to another embodiment, thecurrent, power and energy information may be provided to an externalclient through another wired type of interface, such as en Ethernet orPower Line Communication (PLC) port.

In one embodiment described above, each smart sensor circuit 120determines power information for its associated branch circuit andtransmits the information to the CT concentrator 124. In anotherembodiment, which will now be described with reference to FIG. 6, the CTconcentrator 124 synchronizes current measurements by each smart sensorcircuit 120 with voltage measurements performed by the CT concentrator124. This allows the CT concentrator 124 to calculate power informationbased only on current information received from the smart sensorcircuits 120.

FIG. 6 is a flow chart of a method of operation of the CT concentrator124 of FIG. 5, according to one embodiment. At block 602, the CTconcentrator 124, and hence the smart sensor circuits 120, are poweredup. At block 604, the primary microcontroller 528 of the CT concentratorassigns unique addresses to each smart sensor circuit 120, via thecommunications bus 122. According to one embodiment, the primarymicrocontroller 528 utilizes an auto addressing scheme, as discussedabove. At block 606, the primary microcontroller 528 transmits controlinformation to each smart sensor circuit 120, via the communication bus122. According to one embodiment, the control information includes atleast one of frequency (or period), the number of samples per period,and a defined sleep timer. In another embodiment, the controlinformation includes scaling parameters. According to anotherembodiment, the control information includes previous cycle computationresults (e.g., for RMS current, power, energy).

At block 608, the primary microcontroller 528 requests each smart sensorcircuit 120 to acknowledge the receipt of the control information viathe communication bus 122. According to one embodiment, at block 608,the primary microcontroller 528 also requests that each smart sensorcircuit 120 transmit its sensor type (e.g., 20 A, 80 A, or 200 A currenttransformer) to the primary microcontroller 528 via the communicationbus 122. At block 610, the primary microcontroller 528 creates aninventory of all of the sensor circuits 120 and their type (e.g., bymodel number). At block 612, the primary microcontroller 528 transmitsto each smart sensor circuit 120 that the smart sensor circuit 120should enter power save mode.

According to one embodiment, once a smart sensor 120 enters power savemode, a sleep timer is enabled. In one embodiment, the use of the sleeptimer is intended to limit the overall power consumption of the system.For example, in one embodiment, when a smart sensor 120 is in power savemode, the smart sensor 120 will not communicate on the communicationbus, and hence will require a lower level of power, until the sleeptimer has expired. By placing at least a portion of the smart sensors120 in power save mode, the total number of smart sensors 120 requiringfull power is limited and the total peak power consumption of the systemmay be reduced. According to one embodiment, the sleep timer isprogrammable. In one embodiment, the sleep timer is configured with atime equal to slightly less than the total number of smart sensors 120multiplied by the period over which current is to be sampled.

For example, according to on embodiment, the sleep timer is configuredwith a time (T) calculated with the following formula:

T=(s−2)*t+(t/2);

where:

s represents the total number of smart sensors 120, and

t represents the sample period defined by the primary microcontroller528.

In one example, where the sample period is 20 ms and the system includesa total of 6 smart sensors 120, the time T is calculated as 90 ms. Inthis example, after a smart sensor 120 has conducted measurements andfinished transmitting current sample raw data, it will enter power savemode for 90 ms and will not sample current again until time T (90 ms)has expired. However, in other embodiments, the sleep timer may beconfigured differently.

In one embodiment, smart sensors 120 currently in power save mode areconfigured to exit power save mode early (i.e., before the expiration oftime T), to prepare for current sampling which will begin upon theexpiration of time T. For example, in one embodiment, smart sensors 120currently in power save mode are configured to exit power save mode 10ms early. In such an embodiment, the total time each smart sensor 120will be awake is 30 ms (20 ms period in addition to 10 ms awakeningperiod). Hence, by staggering the current sampling performed by thesmart sensors 120, the number of smart sensors 120 requiring power atthe same time is limited and as a result, the total peak powerconsumption of the system is reduced. This is particularly useful forbattery operated systems.

According to another embodiment, rather than utilizing a sleep timer, asmart sensor 120 exits power save mode upon detecting traffic on thecommunication bus 122.

At block 614, the primary microcontroller 528 senses the voltage,frequency and/or phase of the power signal information received from thepower module 126 via the sensor interface 520. For example, according toone embodiment, the primary microcontroller 528 senses voltage and/orfrequency through a voltage sense signal and the primary microcontroller528 senses phase through a phase synchronization signal. As discussedabove, according to some embodiments, the power signal informationreceived from the power module 126 may be correlated to single, doubleor 3-phase power.

At block 616, the primary microcontroller 528 computes the RMS voltagefor all phases that are present (e.g., 1, 2, or 3). Also at block 616,the primary microcontroller 528 compares the RMS voltage to the primarymicrocontroller's 528 nominal voltage to confirm that the RMS voltageand phase signal(s) are correct. For example, according to oneembodiment, if the primary microcontroller 528 is connected to a utilitysystem in North America, the primary microcontroller 528 will confirmthat it is measuring a 120V, 60 Hz signal. However, in anotherembodiment, if the primary microcontroller 528 is connected to a utilitysystem in Europe, the primary microcontroller 528 will confirm that itis measuring a 220V, 50 Hz signal.

At block 618, the primary microcontroller 528 determines the appropriatephase angle at which synchronized measurements will be taken. Accordingto one embodiment, the phase angle may be configured as any phase angle,and does not have to be limited to a zero crossing. In some embodiments,the phase angle may be configured at an angle other than at a zerocrossing to intentionally avoid noise which may exist at the zerocrossing.

At blocks 620 and 622, synchronized sampling by the primarymicrocontroller 529 and the smart sensor circuits 120 begins at thepreviously determined phase angle. For example, according to oneembodiment, at block 620, the primary microcontroller 528 communicatesto all of the smart sensor circuits 120 simultaneously via thecommunication bus 122 to start sampling current in their respectivecircuit branches 102 at the predetermined phase angle. Also, at the sametime as block 620, the primary microcontroller 528 at block 622initiates voltage sampling of the power signal information received fromthe power module 126 at the previously determined phase angle tosynchronize the voltage measurements with the current measurements madeby all of the smart sensor circuits 120. According to one embodiment,the primary microcontroller 528 samples voltage over the same period oftime in which the smart sensor circuits 120 sample current.

According to another embodiment, instead of communicating to all of thesmart sensor circuits 120 simultaneously, the primary microcontroller528 communicates to at least one specific sensor (e.g., a sensor havinga unique address) to begin sampling current in the respective circuitbranch 102. In this way, the primary microcontroller 528 is able tostart sampling current in at least one specific type of circuit branch(e.g., a circuit branch coupled to a specific type of load). By onlysampling current in a select number of circuit branches 102, the overallpower consumption of the system may be reduced.

According to one embodiment, each smart sensor circuit 120 which iscontrolled to begin sampling will sample current in the smart sensorcircuits 120 respective branch over a predefined period of time for apredefined number of samples, the time and number of samples beingpreviously set by the primary microcontroller 528 in the controlinformation. In one embodiment, the current sampling raw data is storedin a buffer of each smart sensor circuit 120.

At block 624, upon completing voltage sampling for the given period, theprimary microcontroller 528 requests that each smart sensor circuit thatwas sampling current, transmit the current sampling raw data for thegiven time period from the buffer to the primary microcontroller 528 viathe communication bus 122. According to one embodiment, the currentsampling raw data is time-stamped.

At block 626, upon confirming receipt of the current sampling raw data,the primary microcontroller 528 transmits to the previous currentsampling smart sensors 120 that the smart sensors 120 should enter powersave mode, making more power available for other smart sensors (asdiscussed above).

According to one embodiment, at block 626, using the received currentdata and measured voltage data, the primary microcontroller 528calculates the RMS current, real power (e.g., 4 quadrant), and/or energyusage of the circuit branches 102 associated with the smart sensors 102from which the primary microcontroller 528 received the raw currentsampling data. According to one embodiment, the primary microcontroller528 may automatically account for any communication delay between theprimary microcontroller 528 and the smart sensor circuits 120 whenmaking its current, power and/or energy calculations. For example, in atleast one embodiment, the primary microcontroller synchronizes the clocktimes of the smart sensor circuits 120 with the clock time of theprimary microcontroller 528 to account for any communication delaybetween the primary microcontroller 528 and the smart sensor circuits120 via the communication bus 122. Synchronization of the clock timesbetween the primary microcontroller 528 and the smart sensor circuits120 to account for transmission latency by the communication bus 122 isdiscussed in greater detail below.

After calculating the current, power and energy information, the primarymicrocontroller 528 may repeat blocks 620 to 628 for another smartsensor 120 or group of smart sensors 120.

In at least some embodiments, the use of the primary microcontroller 528to individually control the synchronization of the smart sensor circuits120, eliminates any need to individually wire each smart sensor circuit120 with phase synchronization signals from the power module. PhaseLocked Loop (PLL) circuitry within the smart sensor circuits 120 mayalso be eliminated, as the primary microcontroller 528 will control thesynchronization. By allowing the primary microcontroller 528 to selectthe phase angle at which sampling will occur, the flexibility of thesystem may be increased. For example, any appropriate phase angle may beselected to provide the most desirable results.

FIG. 7 is a flow chart 700 illustrating a method of clocksynchronization between the primary microcontroller 528 of the CTconcentrator 124 and smart sensor circuits 120 in accordance withaspects of the present invention. According to one embodiment, theclocks of the primary microcontroller 528 and the smart sensor circuits120 include second, millisecond, and microsecond counters; however, inother embodiments, the clocks may include lesser or greater resolution.

At block 702, the primary microcontroller 528 increments a countervariable (C). At block 704, the primary microcontroller 528 starts anelapse timer. According to one embodiment, the elapse timer hasmicrosecond scale resolution; however, in other embodiments, theresolution of the elapse timer may be configured differently.

At block 706, upon starting the elapse timer, the primarymicrocontroller 528 transmits a relatively short message (SetSecond) toa smart sensor circuit 120. The SetSecond message is configured tosynchronize a second counter of the smart sensor circuit 120 to a secondcounter of the primary microcontroller 528. In one embodiment, where theprimary microcontroller 528 utilizes the Modbus serial communicationprotocol to communicate with the smart sensor circuits 120 via thecommunication bus 122, the primary microcontroller 528 utilizes FunctionCode 6 of Modbus to transmit an eight byte “Write Single Register”command to the smart sensor circuit 120 that writes a single sixteen bitvalue to the smart sensor circuit 120.

In one embodiment, where the primary microcontroller 528 is transmittingthe SetSecond message to the smart sensor circuit 120, the primarymicrocontroller 528 places the first byte of the SetSecond message inits transmit buffer. Once the entire first byte of the SetSecond messageis transmitted to the smart sensor circuit 120 and the transmit bufferof the primary microcontroller 528 is empty, an interrupt serviceroutine automatically sends the remaining bytes of the SetSecond messageto the smart sensor circuit 120.

At block 708, the smart sensor circuit 120 receives the SetSecondmessage from the primary microcontroller 528. Once the smart sensorcircuit 120 identifies a frame mark condition on the communication bus122 (e.g., bus inactivity for 3.5 bits), the smart sensor circuit 120recognizes that it has received the entire SetSecond message. At block710, the smart sensor circuit 120 decodes the received SetSecond messageand utilizes the decoded sixteen bit value to set its sixteen bit secondcounter. In other embodiments, the second counter may be of a differentsize (e.g., an eight bit or thirty-two bit counter). Also, in otherembodiments, different protocol types or sized signals may be used bythe primary microcontroller 528 to set the second counter of a smartsensor circuit 120.

At block 712, the smart sensor circuit 120 replies to the SetSecondmessage from the primary microcontroller 528 that it has successfullyreceived the SetSecond message. In one embodiment, the smart sensorcircuit 120 places the first byte of the reply to the SetSecond messagein its transmit buffer. Once the entire first byte of the replay istransmitted to the primary microcontroller 528 and the transmit bufferof the smart sensor circuit 120 is empty, an interrupt service routineautomatically sends the remaining bytes of the reply to the SetSecondmessage to the primary microcontroller 528.

At block 714, the primary microcontroller 528 receives the reply to theSetSecond message from the smart sensor circuit 120. Once the primarymicrocontroller 528 identifies a frame mark condition on thecommunication bus 122 (e.g., bus inactivity for 3.5 bits), the primarymicrocontroller 528 recognizes that it has received the entire reply tothe SetSecond message from the smart sensor circuit 120. At block 715,in response to recognizing that it has received the entire SetSecondreply from the smart sensor circuit 120, the primary microcontroller 528stops the elapse timer and records the current value of the elapsetimer. The recorded elapse timer value represents the total time thathas elapsed since the SetSecond message was transmitted by the primarymicrocontroller 528 to the smart sensor circuit 120 and is indicative ofthe total Round Trip Time (RTT) required for a message to be transmittedfrom the primary microcontroller 528 to the smart sensor circuit 120 andfor a reply to be returned to the primary microcontroller 528 from thesmart sensor circuit 120.

As discussed above, the initial relatively short message (SetSecond)sent from the primary microcontroller 528 to the smart sensor circuit120 is utilized for both RTT measurement and second counterconfiguration purposes. However, in another embodiment, the initialrelatively short message sent from the primary microcontroller 528 tothe smart sensor circuit 120 may only be used for RTT measurementpurposes and not for counter configuration purposes.

At block 716, the primary microcontroller 528 determines if the countervariable (C) equals a predefined number of measurements (N). In responseto a determination that the counter variable (C) does not equal thepredefined number of measurements (N), at block 702 the counter variable(C) is incremented, at block 704 the elapse timer is reset andrestarted, and at blocks 706-715 the primary microcontroller 528calculates another RTT value, as discussed above.

At block 718, in response to a determination that the counter variable(C) equals the predefined number of measurements (N), the primarymicrocontroller 528 calculates a representative RTT value based on thepreviously calculated RTT values. For example, in one embodiment, theprimary microcontroller 528 calculates a representative RTT value bycalculating the median value of previously calculated RTT values. Inother embodiments, the representative RTT value may be calculateddifferently (e.g., with an average RTT value, a maximum RTT value,etc.).

At block 720, the primary microcontroller 528 utilizes half of therepresentative RTT value as a clock offset to synchronize a millisecondcounter of the smart sensor circuit to a millisecond counter of theprimary microcontroller 528. The primary microcontroller 528 sets themillisecond counter of the smart sensor circuit to a value thatcorresponds to a value of the millisecond counter of the primarymicrocontroller 528 but that is also adjusted to account for the timerequired for a message to pass from the primary microcontroller 528 tothe smart sensor circuit 120 (i.e., the millisecond time of the primarymicrocontroller 528 plus half of the representative RTT). At block 722,the primary microcontroller 528 utilizes half of the representative RTTvalue as a clock offset to synchronize a microsecond counter of thesmart sensor circuit to a microsecond counter of the primarymicrocontroller 528. The primary microcontroller 528 sets themicrosecond counter of the smart sensor circuit to a value thatcorresponds to a value of the microsecond counter of the primarymicrocontroller 528 but that is also adjusted to account for the timerequired for a message to pass from the primary microcontroller 528 tothe smart sensor circuit 120 (i.e., the microsecond time of the primarymicrocontroller 528 plus half of the representative RTT).

For example, in one embodiment, where a fourteen bit millisecond counteris utilized by the smart sensor circuit 120, the primary microcontroller528 first utilizes Function Code 6 of Modbus to transmit a “Write SingleRegister” command to the smart sensor circuit 120 that writes a singleten bit value (corresponding to the value of the primarymicrocontroller's millisecond counter adjusted by half of therepresentative RTT) to the smart sensor circuit 120. The smart sensorcircuit 120 utilizes the ten bit value received from the primarymicrocontroller 528 to set the first ten bits of its millisecondcounter.

The primary microcontroller 528 then utilizes Function Code 6 of Modbusto transmit a “Write Single Register” command to the smart sensorcircuit 120 that writes a four bit value and a ten bit value to thesmart sensor circuit 120. The smart sensor circuit 120 utilizes the fourbit value to set the lower nibble of its millisecond counter to a valueadjusted by half of the representative RTT (as discussed above) andutilizes the ten bit value to set its microsecond counter to a valuecorresponding to the value of the primary microcontroller's microsecondcounter adjusted by half of the representative RTT. Accordingly, thecombined twenty bits of the smart sensor circuit's 120 millisecond andmicrosecond counters are synchronized with the millisecond andmicrosecond counters of the primary microcontroller 528 to account fortransmission latency in the communication bus 122.

As described above, two messages are used to set the combinedtwenty-four bits of the smart sensor circuit's millisecond andmicrosecond counters as Modbus Function Code 6 messages may onlytransmit single sixteen bit words. However, in other embodiments, moreor less than two messages may be utilized to set the counters of a smartsensor circuit. Also, the messages used to set the counters of a smartsensor circuit may be of any appropriate size and/or type protocol.Additionally, the counters of the smart sensor circuit 120 and/or theprimary microcontroller 528 may be configured differently or be of anyappropriate size.

The primary microcontroller 528 may repeat the clock synchronizationprocess described above (with regard to FIG. 7) over multiplesynchronization cycles to maintain accurate synchronization between theclock of the primary microcontroller 528 and the clock of the smartsensor circuit 120. The primary microcontroller 528 may also utilize thesynchronization process described above (with regard to FIG. 7) toindividually synchronize the clocks of any number of smart sensorcircuits 120 within the load center 100 to the clock of the primarymicrocontroller 528 to account for latency in the communication bus 122.

In one embodiment, by utilizing the primary microcontroller 528 toinitiate and control clock synchronization of the smart sensor circuits120 and the primary microcontroller 528 to account for transmissionlatency in the communication bus 122 (e.g., an RS-485 communication bus)as discussed above, the clocks of the smart sensor circuits 120 and theclock of the primary microcontroller 528 may be synchronized to lessthan 0.1 millisecond of each other. This may ensure accuratesynchronization between voltage measurements made by the primarymicrocontroller 528 and current measurements made by the smart sensorcircuits 120.

According to one embodiment, in addition to adjusting the clocks of thesmart sensor circuits 120 to account for transmission latency over thecommunication bus 122, the primary microcontroller 528 may also adjustthe elapse timer to compensate for smart sensor circuit clock skew.Clock skew occurs when the clock of a smart sensor circuit 120consistently runs either faster or slower than the clock of the primarymicrocontroller 528. In one embodiment, the primary microcontroller 528performs an adaptive algorithm that periodically adds or subtracts acorrection factor to/from the elapse timer. The correction factor isiteratively calculated by measuring the skew of the smart sensor circuitclock (i.e., the difference between the smart sensor circuit clock andthe clock of the primary controller 538) at successive clocksynchronization cycles. The correction factor is tuned until a minimumskew is achieved. In one embodiment, the correction factor has amicrosecond scale; however, in other embodiments, the correction factormay be of any other scale.

In another embodiment, in addition to calculating a representative RTTas discussed above, the primary microcontroller 528 may utilizeinterrupt service routines (e.g., at the end of a SetSecond messagetransmitted by the primary microcontroller 528 or at the end of aSetSecond replay transmitted by a smart sensor circuit 120, as discussedabove) with known latencies to further define the representative RTT.For example, if the primary microcontroller 528 recognizes that theactual calculated latency of an interrupt service routine is longer thana predetermined latency for the interrupt service routine, the primarymicrocontroller 528 may use the difference between the actual latencyand the predetermined latency to adjust the clock of a smart sensorcircuit 120 to account for the additional latency within the system.

Even though examples in accordance with the present invention aredescribed herein with reference to a load center, other examples may beutilized within any electrical system in which current, power and energyof a power line are desired to be monitored. It also is to beappreciated that examples in accordance with the present invention maybe utilized to monitor any type (e.g., commercial or residential) orsize system.

As described above, the synchronization process (e.g., as seen in FIG.7) is utilized within a load center to synchronize the clocks of smartsensor circuits to the clock of a concentrator; however, in otherembodiments, the synchronization process may be utilized to synchronizethe clocks of any other types of devices which are coupled together viaa multi-drop bus.

As described above, the synchronization process is implemented over acommunication bus utilizing the Modbus serial communication protocol;however, in other embodiments, the synchronization process may beutilized with any multi-drop bus implementing Modbus, Fieldbus,ProfiBus, or any other Multidrop master-slave protocol using adeterministic hardware network.

Even though examples in accordance with the present invention aredescribed herein as utilizing a current transformer 114 capable of beingclamped onto a circuit branch 102, other examples may utilize adifferent type of current sensor. For example, current sensors utilizingshunt resistance, hall-effect, and toroidal (solid core) currenttransformers may be used.

By including only a single communication bus within a load center,rather than individual dedicated connections (e.g., “hub and spokewiring”), and connecting all smart CT's to a CT concentrator within theload center via the single communication bus; a relatively small, lesscomplex and more manageable method and system for utilizing a pluralityof CT's to monitor circuit branches of a load center is provided. Also,by utilizing the CT concentrator to initiate and control clocksynchronization of the smart CT's and to account for transmissionlatency in the communication bus, as discussed above, the clocks of thesmart CT's and the clock of the CT concentrator may be synchronized toless than 0.1 millisecond of each other. As each smart CT is interruptdriven and in communication with the CT concentrator via the samecommunication bus (i.e., utilizing the same communication protocol), thesmart CT's are able to respond quickly to the CT concentrator and havesubstantially the same transmission latency for messages from the CTconcentrator.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

What is claimed is:
 1. A system for monitoring a plurality of circuitbranches coupled to an input line, the system comprising: acommunication bus; a controller having a primary clock with a firstclock value, the controller configured to be coupled to thecommunication bus and the input line and further configured to samplevoltage on the input line based on the first clock value; a plurality ofsensor circuits, each sensor circuit having a secondary clock with asecond clock value and each sensor circuit configured to be coupled tothe communication bus and at least one of the plurality of circuitbranches, wherein each sensor circuit is further configured to samplecurrent in the at least one of the plurality of circuit branches basedon the second clock value; and wherein the controller is furtherconfigured to initiate, via the communication bus, synchronization of atleast one secondary clock and the primary clock, and to synchronize, viathe communication bus, the at least one secondary clock and the primaryclock to account for transmission latency in the communication bus. 2.The system of claim 1, wherein the controller is further configured toutilize a multi-drop master-slave communication protocol to communicatewith the plurality of sensor circuits via the communication bus.
 3. Thesystem of claim 1, wherein in initiating synchronization of the at leastone secondary clock and the primary clock, the controller is furtherconfigured to transmit a measurement signal to at least one sensorcircuit having the at least one secondary clock and to start a timerhaving an elapsed time value upon transmitting the measurement signal.4. The system of claim 3, wherein the at least one sensor circuit isfurther configured to receive the measurement signal and transmit aresponse to the measurement signal to the controller.
 5. The system ofclaim 4, wherein the at least one sensor circuit is further configuredto adjust the second clock value of the secondary clock based on themeasurement signal.
 6. The system of claim 4, wherein the controller isfurther configured to receive the response to the measurement signalfrom the at least one sensor circuit, stop the timer in response toreceiving the response to the measurement signal, and calculate at leastone Return Trip Time (RTT) based on the elapsed time value of the timer.7. The system of claim 6, wherein the controller is further configuredto calculate a representative RTT based on the at least one RTT.
 8. Thesystem of claim 7, wherein the representative RTT is one of a medianRTT, mean RTT, and maximum RTT.
 9. The system of claim 7, wherein thecontroller is further configured to transmit a first synchronizationsignal based on the representative RTT to the at least one sensorcircuit, and wherein the at least one sensor circuit is furtherconfigured to adjust the second clock value of the secondary clock basedon the first synchronization signal.
 10. The system of claim 9, whereinthe at least one sensor circuit is further configured to adjust amillisecond counter of the secondary clock based on the firstsynchronization signal.
 11. The system of claim 9, wherein thecontroller is further configured to transmit a second synchronizationsignal based on the representative RTT to the at least one sensorcircuit, and wherein the at least one sensor circuit is furtherconfigured to adjust the second clock value based on the secondsynchronization signal.
 12. The system of claim 11, wherein the at leastone sensor circuit is further configured to adjust a microsecond counterof the secondary clock based on the second synchronization signal. 13.The system of claim 1, wherein the controller is further configured tosynchronize, via the communication bus, current sampling performed bythe plurality of sensor circuits with the voltage sampling performed bythe controller.
 14. A method for monitoring a plurality of circuitbranches coupled to a power line, the method comprising: coupling acontroller to the communication bus and to the power line, thecontroller having a primary clock with a first clock value; coupling asensor circuit to each one of the plurality of circuit branches and to acommunication bus, each sensor circuit having a secondary clock with asecond clock value; sampling, with at least one of the sensor circuits,current in at least one of the plurality of circuit branches based onthe second clock value; sampling, with the controller, voltage on thepower line based on the first clock value; and synchronizing, with thecontroller via the communication bus, the at least one secondary clockand the primary clock to account for transmission latency in thecommunication bus.
 15. The method of claim 14, wherein synchronizing theat least one secondary clock and the primary clock includes: calculatingat least one RTT from the controller to at least one sensor circuithaving the at least one secondary clock; and transmitting at least onesynchronization signal from the controller to the at least one sensorcircuit to adjust the second clock value of the secondary clock of theat least one sensor circuit based on the at least one RTT.
 16. Themethod of claim 15, wherein calculating at least one RTT includes:transmitting a measurement signal from the controller to the at leastone sensor circuit; in response to transmitting the measurement signal,starting a timer of the controller having an elapsed time value;receiving, with the at least one sensor circuit, the measurement signal;transmitting, in response to receiving the measurement signal, aresponse to the measurement signal from the at least one sensor circuitto the controller; receiving, with the controller, the response to themeasurement signal; stopping, in response to receiving the response tothe measurement signal, the timer of the controller; and calculating theat least one RTT based on the elapsed time value of the timer.
 17. Themethod of claim 16, further comprising calculating a representative RTTbased on a plurality of RTT calculations, and wherein transmitting atleast one synchronization signal from the controller to the at least onesensor circuit includes transmitting at least one synchronization signalfrom the controller to the at least one sensor circuit to adjust thesecond clock value of the at least one sensor circuit based on therepresentative RTT.
 18. The method of claim 14, further comprisingsynchronizing, with the controller via the communication bus, currentsampling performed by the plurality of sensor circuits with the voltagesampling performed by the controller.
 19. The method of claim 14,wherein transmitting at least one synchronization signal from thecontroller to the at least one sensor circuit to adjust the second clockvalue of the at least one sensor circuit based on the at least one RTTincludes utilizing a multi-drop master-slave communication protocol totransmit the at least one synchronization signal from the controller tothe at least one sensor circuit.
 20. A system for monitoring a pluralityof circuit branches coupled to an input line, the system comprising: acommunication bus; a controller having a primary clock having a firstclock value, the controller configured to be coupled to thecommunication bus and the input line and further configured to samplevoltage on the input line based on the first clock value; a plurality ofsensor circuits, each sensor circuit having a secondary clock with asecond clock value and each sensor circuit configured to be coupled tothe communication bus and at least one of the plurality of circuitbranches, wherein each sensor circuit is further configured to samplecurrent in the at least one of the plurality of circuit branches basedon the second clock value; and means for synchronizing, with thecontroller via the communication bus, the secondary clock of at leastone of the plurality of sensor circuits and the primary clock of thecontroller to within 0.1 millisecond.